1. Field of the Invention
The present invention relates to an optical communication apparatus, and more particularly to an optical communication apparatus in which an output waveform of an optical fiber amplifier (refer it simply to as an EDFA, hereinafter) is improved.
2. Description of the Related Art
Meeting the age of multi-media including an Internet, the optical communication network technique of a main communication system in which more improved service is desired over a wide area has been rapidly and progressively developed for an information society. Further, as the capacity of an optical communication line becomes large and the speed of the optical communication line becomes high, a receiving section of an optical transmitting system has been also requested to have a receiving function of a higher performance.
As one of components of the optical communication apparatus, an EDFA (erbium-doped optical fiber amplifier) obtained by adding an erbium ion to an optical fiber has been known.
FIG. 7 is a diagram showing a basic structure of an optical communication apparatus using the EDFA. The optical communication apparatus 1 includes the EDFA2, a light receiving element 3, an amplifier 4 and a signal reproduction section 5.
The EDFA2 receives and amplifies a signal from a modulation light source 10 that is transmitted through an optical fiber cable 6. The light receiving element 3 converts an amplified optical signal to an electric signal. The amplifier 4 amplifies the electric signal. In the signal reproduction section 5, identification of “1” or “0” is performed based on a set threshold value to reproduce data.
The EDFA has excellent features such as high gain, high output, wide band, low noise and independence of a polarized wave. Therefore, the EDFA is indispensable for a present optical fiber communication system. In the present optical communication, the EDFA is employed in a structure in which a light that is modulated by a constant rapid period is inputted to the EDFA.
Related arts using such EDFA are disclosed in JP-A-2003-179552 and JP-A-2002-026874.
However, in the EDFA, when a light is suddenly inputted from a state that the light is not inputted or a pulse-shaped light having a long period equal to or above the order of μs is inputted, a light surge appears that has an output several times to several ten times as large as an output that can be obtained from an ordinary gain or a saturated output. This phenomenon results from a fact that a relaxation time of excitation and emission in the EDFA is in the order of μs. Accordingly, there is a possibility that optical components arranged in a rear stage may be broken due to the light surge.
FIG. 8 shows an input light inputted to the EDFA2 from the modulation light source 10 of the optical communication apparatus 1 shown in FIG. 7. FIG. 9 shows a waveform of an output light from the EDFA2 and a state that the light inputted from the modulation light source 10 is inputted to the EDFA2 and outputted as an amplified and modulated light. In these drawings, an axis of ordinate designates a value (mv) obtained by inputting a light power to a PD (Photo Diode) and converting it to an electric signal, and an axis of abscissa designates a time (μs).
When a rectangular optical signal having a long period (1 kHz, −10 dBmp-p) ΔY=2.0 mv as shown in FIG. 8 is inputted, the output waveform is ΔY=40 mv as shown in FIG. 9, so that a large gain can be obtained. However, in a head portion of the rectangular signal, the light surge having an extremely large output exceeding 100 mv appears as shown in a part A.
FIGS. 10A-10D show output waveforms when the power of the input light is changed from 0 dBmp-p to −3 dBmp-p, to −7 dBmp-p and to −10 dBmp-p. Also in this drawing, an axis of ordinate designates a value (mv) obtained by inputting a light power to a PD and converting it to an electric signal and an axis of abscissa designates a time (μs). The frequency of a signal light at this time is set to 1 kHz. A large protrusion (surge voltage) is generated at the beginning of the waveform, which is different from a case of 10 GHz. This protrusion shows that as the power of the input light (0 dBmp-p to −3 dBmp-p, to −7 dBmp-p and to −10 dBmp-p) is large, the protrusion of the surge voltage is also large.
As described above, in the case of a wavelength of about 1 kHz, the light surge appears, however, when a modulated light in unit of GHz employed in the present optical communication is continuously inputted, the EDFA can be used as an extremely good optical amplifier.
However, in an optical switch using a label switching high in its availability as an optical communication apparatus in future, an optical burst switching network of a next generation or an optical packet switching network, a signal is intermittently inputted. Accordingly, during a time of disconnection, there is a state that the light is not inputted to the EDFA. Therefore, in an ordinary using method, the light surge is undesirably generated as shown in FIG. 9.
When the modulated light of several GHz that is used in the present optical communication is constantly and continuously inputted to the EDFA, the EDFA can be used as a very preferable amplifier. However, when a light is suddenly inputted from a state that the light is not inputted or a pulse-shaped light having a long period equal to or more than the order of μs as shown in FIG. 8 is inputted, a light surge appears that has an output several times to several ten times as large as an output obtained from an ordinary gain or a saturated output.
Accordingly, since in the optical switch using the label switching in future, the optical burst switching network of a next generation or the optical packet switching network, there is a state that the light is not inputted to the EDFA, the light surge is undesirably generated by a current using method.